The invention relates to a ruthenium catalyst on a support, a process for its preparation and a process for the preparation of substituted or unsubstituted cyclohexylamine and substituted or unsubstituted dicyclohexylamine by catalytic hydrogenation of substituted or unsubstituted aniline using such a catalyst.
It is known to prepare cyclohexylamine by hydrogenation under pressure of aniline. For this hydrogenation, cobalt catalysts containing a basic additive (GB No. 969,542) and also Raney cobalt (JP No. 68/03180) are used. According to U.S. Pat. No. 3,636,108, an alkali-moderated ruthenium catalyst on an inert support material is used for the ring hydrogenation of aromatic amino compounds, NH.sub.3 and, if appropriate, a solvent being additionally used. A further process for the hydrogenation under pressure of aniline to give cyclohexylamine is described in DE-AS (German Published Specification) No. 1,106,319, in which likewise a ruthenium catalyst is used. In this process dicyclohexylamine which is also formed is added again to the starting material; the process is accompanied by significant losses caused by the simultaneous formation of cyclohexane. In contrast to the publications mentioned so far, EP No. 53,818 considers palladium-supported catalysts more favourable for the hydrogenation under pressure of aniline than ruthenium catalysts.
In the known hydrogenation processes under pressure of aniline, dicyclohexylamine is formed in addition to cyclohexylamine merely as a byproduct. To obtain dicyclohexylamine in larger amounts, it is prepared by separate processes. Thus, it can be produced, for example, by hydrogenation under pressure of diphenylamine using a ruthenium/Al.sub.2 O.sub.3 catalyst (DE-AS (German Published Specification) No. 1,106,319). Furthermore, dicyclohexylamine is formed in the reaction of cyclohexanone with cyclohexylamine in the presence of palladium on carbon at a hydrogen pressure of 4 bar (FR No. 1,333,692). In a complicated process, dicyclohexylamine can be recovered from the hydrogenation product of aniline over a nickel catalyst by fractionated condensation. Some of the ammonia which is also formed is removed from the remaining mixture, and the residue is recycled in the reaction (German Patent Specification No. 805,518).
A common problem of all processes for the ring hydrogenation of aromatic amines is that in some cases there is a significant formation of cyclohexane as a waste product which cannot be used further. It was therefore desired to develop a novel process which is useful even on an industrial scale, by which both cyclohexylamine and dicyclohexylamine can be prepared in one reaction step in a desired ratio of amounts, in which the loss caused by the formation of cyclohexane is reduced and in which furthermore the life of the catalyst used is improved.